(CNN) -- Aaron Peirsol was the victim of the biggest shock of the world swimming championships so far when he failed to qualify for the final of the 100 meters backstroke in Rome.
Defending world champion Peirsol reflects on missing out on the final of the men's 100m backstroke.
The 26-year-old is the only man to have gone under 52 seconds for the event and is the winner of the last three world titles.
The American has also won seven Olympic medals, five of them gold, as well as seven world titles and one silver -- and holds the world records in both the 100m and 200m backstroke.
But, lying second at halfway in Monday's semifinal, Peirsol miscalculated on the second length to find himself in fourth in 53.22 seconds and ninth overall -- missing out on a final place.
Peirsol admitted he had made a major error, telling reporters: "I thought I was going faster. The result has surprised me.
"I am very disappointed; I completely misjudged my race. I have to go on, pick myself up and focus on my next event."
Five world records were set in the evening session at the Foro Italico, with the first coming in the opening event.
Australian Brenton Rickard set a new record in the 100m breaststroke, with two men dipping under Kosuke Kitajima's old mark.
Rickard pipped France's Hugues Duboscq, who was seventh into the final, in 58.58 to take 0.33 off the old mark. Duboscq also bettered the old record in 58.64 -- with Cameron Van Der Burgh of South Africa third in 58.95.
Eric Shanteau, who competed in the Beijing Olympics despite having been diagnosed with testicular cancer shortly beforehand, was touched out into fourth.
The second event then saw Swedish teenager Sarah Sjostrom set her second 100m butterfly world record in 24 hours.
Gabriella Silva was 0.71 under world record pace at halfway, with Sjostrom in eighth. Jessicah Schipper then powered down the final length but was overhauled by the 15-year-old in the final meters to touch in 56.06 -- 0.38 inside the mark she set on Sunday.
Australian Schipper also went under the old mark in 56.23 to take her second successive world championship silver medal in this event.
Ariana Kukors also set another world record in just 24 hours as she took the 200m individual medley title. She lowered her own mark to 2:06.15, meaning she had taken 2.30secs off Stephanie Rice's old mark over two days. Rice was second, matching the American's record from Sunday, with Hosszu in third.
American Rebecca Soni consigned Leisel Jones' record to history in the semis of the 100m breaststroke, dipping 0.25 inside in 1:04.84. Jones is not competing in Rome, instead choosing to finish her beauty school studies in Australia.